1. Field of the Invention
The present invention relates to an underwater pelletizing apparatus for cutting a hot melt thermoplastic material being extruded out from an extruding machine, a geared pump, and so forth, into water, in a form of pellets by a cutting knife.
2. Description of the Related Art
As this type of pelletizer, the apparatus shown in FIG. 4 is generally known. This apparatus is exchangeably installed on the front end of an extrusion opening section 42 of an extruding machine 41 having a screw shaft 40, etc. for mixing and extruding a thermoplastic material such as a synthetic resin, etc.
In this apparatus a die plate 43 and a water chamber casing 44 are removably mounted on the forward end of the extrusion opening section 42; a cutting knife 45 is disposed at the front of the die plate 43; a cutting knife shaft 46 with the cutting knife 45 mounted on the forward end is rotatably and axially movably supported in a water chamber casing 44 and a shaft supporting housing 47, so that the cutter shaft 46 is moved backward and forward by means of a spring-loaded hydraulic cylinder 48.
An underwater pelletizing apparatus previously adopted is of a press-to-cut system that, when a thermoplastic material is cut under water, the cutting knife 45 is rotated, being pressed with a specific pressure against the die plate 43, to cut the thermoplastic material to a specific size. In this apparatus a pressure oil is suppled into the hydraulic cylinder 48 through a regulator 49, to thereby turn the cutting knife 45 with the cutting knife 45 held pressed against the die plate 43 with a specific pressure.
The pressure for pressing the cutting knife 45 against the die plate 43 is an operating oil pressure to be supplied to the hydraulic cylinder 48 after regulation by means of the regulator 49, rotating the cutting knife 45 in the state that the amount of pressure is kept constant as shown in FIG. 5.
In the above-described example of a prior-art technique, the cutting knife 45 is operated while being constantly pressed against the die plate 43; therefore it is required to press the cutting knife 45 with the least possible pressure for the purpose of decreasing wear of the knife; furthermore, in the event of little pressure, the cutting knife 45 will be pushed backward with the resin being extruded from the die plate 43, to thereby cause the cutter shaft 46 to vibrate, resulting in unstable cutting. It is, therefore, necessary to continuously press With a fixed pressure or a greater pressure indicated by Pmin in FIG. 5.